In my experience, it is a common problem for taiji beginners to feel strain in the knee because the rules of Taijiquan say we should “rotate our hips”. When the kua (the technical term for the hip area) is not open enough, the knees tend to overcompensate and end up rotating. But the knees can only bend and unbend, they aren’t meant to rotate, so when they rotate, it leads to injury.

This can be prevented by paying attention to alignment. One simple way doing this is to keep the knee on top of the foot. Strain comes when the knee goes too far either on one side or on the other, or if it goes forward past the toes.

 At a beginner level, the issue is potential injury. At a more advanced level,  the issue becomes an inability to generate power

When I first started taiji training with my teacher Kee Hong, he would always say: if you want power,  you have to sink your weight.  In fact, I would observe his center of gravity sink whenever someone pushed him. I would also feel his push getting “under me” whenever I pushed on him.

The only thing I could understand from this was that I had to bend my knees more to lower my center of gravity. I thought it was a sure bet that I would be able to issue some sort of power. This shouldn’t be too hard, I told myself, with more than a decade of dedicated Shaolin training with heavy emphasis on stances — I should definitely have the leg strength to be able to bend my knees more.

But I was absolutely unable to do any of what my teacher and his teacher was doing. In fact, it felt like when I would lower my center of gravity, it would make my training partner more stable. After a while, things started to make sense: when I would try to lower my center, I was bending the ankle of my rear foot rather than the knee. This had many negative effects:

1. Put stress on my ankle.
2. Put stress on my knee, that would easily lead to  bad alignment (on this, see Knee Problems? The Importance of Knee Alignment).
3. Lead to “tossing” in the knee .
4. Tend to make my spine bend forward, compromising the central axis of the body.

I started observing my teacher, his classmates and his teacher more closely. I noticed that the most advanced practitioners would not bend their ankles significantly, but would rather only bend the knee without bending the ankle. I started experimenting with this, and noticed some remarkable difference. I shifted my focus away from the joint, and more on bringing the thigh at a 90 degree angle to the lower leg. Many drills that Kee Hong and Chen Zhonghua had taught me started producing results. Through this, I think I managed to start activating the hinge-like action, at least in the rear hip. I could finally stay “between my feet” when I was lowering my center of gravity.